No effect of gender on brain metabolism was found in young healthy subjects. Analysis of high resolution positron emission tomography (PET) data in dementia of the Alzheimer type (DAT) replicated findings of relative metabolic deficits in association neocortex, and increased the ability to see differences in absolute rates. Four subgroups of DAT proteins were identified based on metabolic patterns. Activation of regional cerebral blood flow in occipitotemporal cortex was equivalent during face perception in DAT patients and controls; patients also had frontal activation. Patients with late onset depression had increased brain metabolism in orbitofrontal cortex. A patient with autopsy proven Parkinson's disease had a metabolic pattern indistinguishable from that seen in DAT. Patients with vascular dementia had decreased subcortical glucose metabolism compared to DAT patients. Density of neurofibrillary tangles, but not senile plaques in postmortem brain, was correlated with metabolic reductions. A nonretarded patient with mosaic translocation trisomy 21 had cognitive and metabolic impairments typical of DAT suggesting that genetic determinants of dementia in Down syndrome differ from those responsible for retardation. Patients with trichotillomania, a disease similar to obsessive-compulsive disorder, had higher glucose metabolic rates than to controls.